“The latest direct radio frequency (RF)-sampling transceivers C include Texas Instruments’ AFE7444 and AFE7422 devices, which support four and two antenna channels, respectively. C provide a variety of powerful features that enable a variety of advanced system features such as multi-band and Multi-mode operation, as well as variable frequency and fast frequency hopping are possible. These functions are becoming increasingly common in terms of system concepts such as multi-function arrays, where different sub-arrays of large phased array antennas can be configured to perform multiple functions depending on the situation or mission needs; this includes radar, communications or Electronic warfare (EW ) function, as shown in Figure 1.
The latest direct radio frequency (RF)-sampling transceivers C include Texas Instruments’ AFE7444 and AFE7422 devices, which support four and two antenna channels, respectively. C provide a variety of powerful features that enable a variety of advanced system features such as multi-band and Multi-mode operation, as well as variable frequency and fast frequency hopping are possible. These functions are becoming increasingly common in terms of system concepts such as multi-function arrays, where different sub-arrays of large phased array antennas can be configured to perform multiple functions depending on the situation or mission needs; this includes radar, communications or electronic warfare (EW ) function, as shown in Figure 1.
Multifunctional Phased Array System
In addition, these systems often require fast frequency hopping to gradually adjust to the operating frequency through repetitive or arbitrary sequences, as shown in Figure 2. Doing so can avoid jamming, prevent signal detection, or facilitate the implementation of anti-spoofing techniques (spoofing: tampering with the electronic signature of radar-reflected signals).
Frequency-Agile Operation Across Multiple Nyquist Zones
To gain a better understanding of these capabilities, let’s first examine the functional blocks of an integrated RF sampling transceiver, as shown in Figure 3.
Functional Blocks of the AFE7444/AFE7422 RF Sampling Transceivers
When the receiver is used in conjunction with the transmitter, these functional blocks provide enhanced functionality in the following ways:
・Operates across an extremely wide range of RF frequencies from a few MHz up to 6 GHz, handling a very wide non-transient bandwidth up to 1.5 GHz.
・ A digital signal processing module that supports aggregation and de-aggregation of multiple subbands or waveforms, each subband or waveform can be processed as an independent digital data stream on the receiving or transmitting side.
Multi-band or multi-mode signal processing
Now let’s consider the use case for processing multi-band or multi-mode signals by leveraging broadband sampling, synthesis, and digital processing capabilities. As shown in Figure 4.
Multiband Transmit and Receive Configurations Using AFE7422 and AFE7444
This setup produces a multiband signal that includes three distinct subbands with a total bandwidth of 2.75 GHz. The receiver samples across the entire frequency band spanning multiple Nyquist zones and then feeds the sampled data to a digital downconversion block (with multiple parallel stages). The method is to select multiple subbands and convert them to the fundamental frequency signal through independent digitally controlled oscillators (NCOs) and digital mixers. Apply decimation, then reduce the output sample rate and suppress out-of-band impairments based on the bandwidth of the individual signals.
Conversely, on the transmit side, the individual digital input streams are fed into a number of parallel digital upconversion stages that convert the fundamental frequency signal to its corresponding target frequency. The data is then oversampled to the RF digital-to-analog converter (DAC) output sample rate, and a combined broadband signal (ranging from 700 MHz to 3.45 GHz) is synthesized by the RF DAC in the final stage.
Frequency conversion and frequency hopping
You can extend the previous case by selecting only a single frequency band, utilizing the internal digital loopback, and then applying a frequency shift to the selected subband before retransmitting that signal. As shown in Figure 5.
Frequency Conversion or Frequency Hopping Using the AFE7444/AFE7422
This setup captures the multiband signal described earlier. The digital downconversion block selects an independent subband, converts it to a baseband signal and passes it through a digital filter. Digital filters remove out-of-band impairments such as harmonics or mixing products. An on-chip digital loopback path that allows the digital output data of the digital receiver to be fed directly into the transmitter path without leaving the chip and without having to connect any additional processing equipment.
Simply upconverting the filtered signal back to the originally received frequency creates an on-chip digital repeater. In order to deploy a frequency hopping transmitter, the NCO in the transmitter section needs to be programmed to output the new frequency required, and then the frequency shifted signal is retransmitted. This is shown by the yellow trace in the spectrum analyzer plot in Figure 5 and compared to the originally received multiband spectrum (green trace).
Frequency transitions on oscillators
So far, I’ve exemplified the basic concepts, and similar approaches can be used to support other use cases, including:
・Multi-band frequency conversion. Because of the use of multiple digital downconversion and upconversion blocks in parallel, you can receive and de-aggregate a multiband signal into multiple independent subband signals, then apply an independent frequency shift to each subband signal, and An internal digital loopback feeds into the transmitter path, retransmitting the subband signal after reaching the new frequency.
・Fast frequency hopping. Since we can reprogram the NCO to get an updated frequency in milliseconds, or alternately use multiple NCOs available on each signal path in a ping-pong mode, it is possible to receive and transmit frequency-agile signals in a repetitive or arbitrary sequence. The transition between these two frequencies is shown in Figure 6.
• Ramp generation/direct digital synthesis mode. The built-in sine wave tone generator for each transmitter supports the generation of frequency ramps and frequency modulated continuous waves (FMCW) commonly used in radar systems.
• Simultaneous wide-band scanning and narrow-band observation. Because each receiver front-end sampling stage can be connected to multiple digital processing stages, you can optionally configure a receive path for wideband mode. Output sampled data spanning the full frequency band of Nyquist and observe non-instantaneous bandwidths up to 1.5 GHz to scan for the presence of any signal. At the same time, you can configure a second path in narrow-band decimation mode, zooming in to accurately analyze all signals detected in wide-band mode.
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